HELIX. 101 



These animals have a distinct and very variously 

 divided vesicula mid- p. ^^ 



tijida, which is want- 

 ing in Succinea, Bu- 

 limics, and other allied 

 genera. 



The young shells Helix cantia 



have the outer whorls 



generally more or less keeled, and the axis is always 

 umbllicated or perforated ; but the perforation is 

 sometimes masked by the reflexion of the outer lip 

 of the adult shell over it. 



This genus is known from Zonites by the thick- 

 ening of the outer lip, from Vitrina by the axis 

 being perforated, from Succinea and Bulimus by the 

 axis being depressed (and not elongate), making the 

 shell subglobose or depressed. 



The animals, at the approach of winter, or in very 

 dry weather in summer, recede into their shell, and 

 secrete a quantity of mucus, which being moulded, 

 as it were, on the retracted part of the mantle which 

 encloses the folded-up foot, forms, when it dries by 

 exposure, a cover to the aperture, which is usually 

 membranaceous, with a triangular perforation over 

 the respiratory hole of the mantle. 



In some species, as Helix Pomatia, the membrane 

 becomes strengthened with a quantity of calcareous 

 matter, which is first deposited on the triangular spot 

 before referred to. In this case, the animal forms 

 several membranaceous coverings, a little distance 

 from one another, within the outer, hard, calcareous 

 one, similar to the membranaceous covering of other 



H 3 



